There are two types of digital coding generally used in communication systems for transmission through a communication medium to a receiver if the information signal is originally digital source data, or has been converted to a digital signal. The first type is usually the initial coding of the signal done to place the signal in a desired or standard format, as, for example, may be appropriate for multiplexing. The second type of coding is usually called channel coding because it is the type of coding designed to resist degradation of the signal quality during transmission through the communication medium.
A common type of channel coding is known as block coding. Block coding will usually consist of a number of bits together with a number of additional bits so related to the others to facilitate error detection and/or correction. Another type of channel coding is convolutional coding in which the check bits are periodically inserted in a continuous data stream.
One of the major problems of the communications art today is to use fully and effectively the communication capacity of every channel, regardless of type, in which high demand may occur. The following examples, while not exhaustive, indicate the dimensions of the problem.
In mobile radio communications, in some geographical markets, demand has grown so rapidly that eve modern cellular techniques have had to be supplemented by other techniques.
Farther afield, in the just-beginning market for using optical fibers in local loops and in communication links not classified as major toll links, it is anticipated that occasional demand will force the use of techniques for more fully and effectively using the bandwidth available on the fiber.
Nevertheless, classical channel coding actually uses more bandwidth for the same net usable information because of the additional check bits inserted.
One set of solutions for the capacity problem involves employing nonbinary digital modulation and coding schemes, or more properly, nonbinary digital modulation combined with channel coding by means of appropriate mapping rules.
For example, quardrature amplitude modulation and phase-shift-keying both lend themselves readily to mapping rules which allow one to combine them with channel coding to give various efficiencies and, ultimately, to effectively utilize the available communication bandwidth of a channel, be it radio or optical fiber.
The class of nonbinary digital signals of primary interest to us is that which employs phase-shift-keying (hereinafter, PSK). The combined modulation and coding schemes are known as trellis coding.
For example, in 4-PSK, a signal can be represented digitally by four discrete phases of the carrier signals, transmitting two bits of information. This scheme has twice the information rate of a 2-PSK signal occupying the same bandwidth. Theoretically, still higher rates of information transfer can be achieved by 8-PSK, or 16-PSK (generalizable as M-ary PSK where M=2, 4, 8, 16, etc). For example, three binary bits would be required to represent the same information as each phase of the 8-PSK signal.
Nevertheless, if one looks at the actual relative phases of the signals generated in PSK, which is most easily done by representing them on a polar-plot of the relative phases, one sees that the 4-PSK system has 90.degree. minimum phase difference between states; and the 8-PSK system has only 45.degree. minimum phase difference between states, and so forth. It is thus desirable to take additional steps to preserve the information in the signal against the random phase variations and random additive noise that can occur in the channel during transmission. Somewhat similarly, carrier phase offset variations that occur in the receiver that degrade a signal are also a common problem in some modems.
One of the common techniques used heretofore to overcome the problem of the effect of random additive noise is a method which can be generally characterized as convolutional coding. In such codes the input information bits are passed through a linear network with finite memory in such a way that corresponding to k.sub.1 input information bits, n.sub.1 output channel bits are generated. The code can be described by a finite state "trellis" where the code memory defines the number of states. The "trellis" description is appropriate because it suggests the incremental nature of the allowed changes between sequential states produced by the use of the coding memory.
This type of convolutional coding is combined with phase-shift-keying, or other non-binary types of modulation, such as quadrature amplitude modulation, in a modulation format chosen both to conserve the use of bandwidth in the channel and to overcome the effects of additive noise. Commonly, this combination of coding and modulation is called trellis coded modulation.
In the past, straight forward coding of 8-PSK proved to have few advantages with respect to 4-PSK in commercial practice. A trellis coded 8-PSK system was developed by G. Ungerboeck as described in his article "Channel Coding with Multilevel/Phase Signals", IEEE Transactions on Information Theory, Vol.IT28, No. 1, pp. 55-67, January 1982. This so-called Ungerboeck coding employed a wiser combination of the convolutional coding output bits and the discrete phase values in the modulation format, in order to gain a portion of the theoretically possible advantages. A combination of 8-PSK with a rate 2/3 convolutional code (i.e., rate=k.sub.1 /n.sub.1 =2/3, in terms of the above description) was done in such a way that a code-related property of alternative signals known as the "minimum Euclidean distance" was improved. Keeping this minimum Euclidean distance as large as feasible tended to protect the signal against degradation during transmission. This advantage is now obtained with coded 8-PSK, as compared to uncoded 4-PSK, (which has a smaller minimum Euclidean distance)without expanding the bandwidth employed.
Various modifications and improvements over Ungerboeck coding (e.g., trellis-coded multilevel/phase-shift-keying types of modulation) have been made over the last few years. For example see the articles (1) L.-F. Wei, "Rotationally Invariant Convolutional Channel Coding with Expanded Signal Space -- Part I: 180.degree., "IEEE Journal on Selected Areas in Communications, Vol. SAC-2, No. 5, pp. 659-671, September 1984; (2) M. Oerder, "Rotationally Invariant Trellis Codes for MPSK Modulation," ICC'85, Chicago, IL,June 1985, pp. 18.1.1-18.1.5; (3) G.Ungerboeck,J. Hagenauer, T. Abdel-Nabi, "Coded 8-PSK Experimental Modem for the INTELSAT SCPC-System," Proceedings ICDSC, 7th International Conference on Digital Satellite Communications, Muenchen, May 1986, pp. 299-304; (4) M. Bertelsmeier, "Modified Coded Octal Phase-Shift-Keying with Improved Carrier-Phase Tracking Ability," IEEE Global Telecommunications Conference, GLOBECOM '86, Houston, TX, December 1986, Conference Record, pp. 38.4.1-38.4.5; and (5) G. Ungerboeck, "Trellis Coded Modulation with Redundant Signal Sets, Part I and Part II, IEEE Communications Magazine, Vol. 25, No. 2, pp. 5-21, February 1987.
Nevertheless, problems still exist with respect to phase sensitivity and to the limited pull-in range (about 22.degree.) of trellis-coded 8-PSK systems which are representative of current multilevel/phase-shift-keying systems. Carrier phase variations at least this large are readily encountered in mobile radio communications, e.g., because of geographical or topographical factors and in other systems having fading channels or unstable oscillators or imperfect carrier recovery or combined effects of the above. The latter three types of problems can occur in modems, and both in mobile radio transmission and in the shorter optical fiber communication links, including local loops, where it is not economically feasible to prevent such problems at their inception. If the phase error is outside the pull-in range or interval, the carrier recovery fails, causing a random-walk situation with a long error burst.
Accordingly, it is one objective of this invention to solve this problem, that is, to provide improved protection against degradation for such a signal.